(Information taken from the slackbot in their Slack-channel) :

As of March 31, 2017, at its peak, there was an estimated 4,161,948 TH/s of hashing power securing the Bitcoin network.  So in order to successfully attack Bitcoin, you would need 51% of that which is 2,122,593 TH/s.  

Also, let's discount the fact that with the amount of money we're talking about here you could just pay to have your own ASIC built out in a fab for even less, but let's just keep it simple using released hardware.  An Antminer S9 provides 14 TH/s @ 3000 USD.  Thus, to achieve that 51%, you would have only needed to acquire approx 151,614 Antminer S9s * 3000 = $454,842,000 USD.  

Now, for an apples to apples comparison, let's assume Bitcoin used Decred's hybrid system and thus we'll use the same coin supply, the same price per coin, and the same PoW hash rate.  As of that same March 31, 2017 date, there were around 16,248,000 bitcoins in circulation at a cost of roughly 1000 USD per coin.  Now, let's go ahead and use some less than favorable numbers an assume there is only 33% stake participation and calculate how much money it would take to attack the network by aiming to acquire 33% of the stake.  

Running the numbers, we can see ((1/0.33 - 1) * 0.33)^3 = 0.29, so you would also need roughly 29% of the hash power in addition to 33% of the stake.  So, 33% of 33% of 16,248,000 coins ~= 1,769,407 * 1000 per coin = 1,769,407,000 USD for the PoS portion.  Now, you also need 29% of the hash power, so 4,161,948 TH/s * .29 ~= 1,206,965 TH/s.  Thus, you would need to acquire approx 86,212 Antminer S9s @ 3000 USD = 258,636,000 USD.

  So, in summary, you would need roughly 455 million USD to attack Bitcoin while you would need roughly 2.03 billion USD to attack Decred.

Decred, Litecoin and Bitcoin. - The road to complete consensus, and how crypto-politics will shape the future of economic democracy.